THE SCATTERING OF A LOW-ANGLE PULSE BEAM FROM SEA-FLOOR VOLUME HETEROGENEITIES

A numerical scattering chamber is used to study the scattering of low- grazing-angle geoacoustic energy from a flat, basaltic seafloor with u nderlying volume heterogeneities of various length scales (0.1 less th an or equal to ka less than or equal to 4, where k is the wave number and a is the length scale of the heterogeneity). The two-dimensional s cattered field is computed by the finite-difference method for an inci dent Gaussian pulse beam at 15 degrees grazing angle (total internal r eflection). For a deterministic model the scattering function is not m onotonic with respect to angle, such as predicted by Lambert's law. Pe aks in the scattering function (up to 10 dB above background scatterin g levels) occur at angles that correspond to forward and backward comp ressional and shear head wave propagation. The scattered field is gene rated by primary diffractions directly into the water column, secondar y diffractions from compressional and shear body waves and interferenc e waves, and tertiary diffractions from interface waves that have conv erted into shear body waves. For the models studied, monostatic backsc atter levels are largest for ka of about 1. The levels are -40 dB for no gradient in the mean subbottom velocity profile and about -48 dB fo r a gradient in the mean profile.